The present invention relates generally to a disc drive system and method. More particularly, the present invention relates to a system and method for screening actuator resonance in a disc drive.
Generally, a disc drive system stores data on rotating media, for example a magnetic disc, and uses read/write heads suspended on a moving armature. The heads read/write the data on the rotating media as the surface of the rotating media moves past them. During track access and read/write operations, actuator resonance can be one of the problems. For example, when the drive performs a seeking operation, a resonance may cause the actuator to oscillate such that the head is not able to stabilize at a desired track center over the media. Such error is often referred to as a position error and can be represented by a Position Error Signal (PES). Typically, when a large oscillating PES is detected, the drive is determined to have a resonance mode and therefore the read/write operation cannot be carried on further.
It is known that all physical parts of the disc drive system possess certain resonant modes, where, if a frequency equal to the resonant mode frequency is applied to the system, the part will resonate. The actuator of the disc drive system also possesses such modes. When the actuator is excited, it can generate a large amount of movement of the actuator arm, in turn a large amount of movement of the read/write heads. This prevents the heads from reading/writing the correct tracks center on the disc surface and may result in the disc drive controller inhibiting a write or read. The inhibiting operation is also known as a write protect/write unsafe or a read protect/read unsafe. When the duration of the disturbance is prolonged, a write or read fault is declared, i.e. the disc drive indicates that it cannot write or read the data, despite repeated attempts. As a result, a fatal error condition occurs.
Due to the ever increasing track densities, Track Per Inch (TPI), required by the disc drive industry, actuator resonance has become one of the major problems seen in disc drive operation. The known types of actuator resonance are slider resonance, arm flexure resonance, and other types of structural resonance. A slider is the suspension which holds heads to the actuator arm. The windage between the head and the rotating disc may cause the slider to oscillate at its resonant frequency which will cause the head not to stay on the track properly, i.e., large oscillating PES. An arm flexure resonance is generally defined as the arm of the actuator, due to the actuator resonance, bending toward or away from the track center substantially such that data cannot be read from the track or written to the track. During manufacturing of a disc drive, it can be quite difficult to determine whether a drive exhibits actuator resonance or resonance in another component.
Recent developments in hard disc drives especially with a higher TPI make the drive""s actuator more difficult in keeping heads on track. The higher the TPI, the smaller the track pitch (i.e., width of a track ). This makes the actuator motor, often referred to a drive""s Voice Coil Motor (VCM), more sensitive to resonance if compared to drives with a lower TPI.
A simple test to verify whether a drive exhibits resonance is to perform a seeking operation. In a drive seeking operation, an acceleration/deceleration current is applied to the VCM depending on its movement. The drive seeks from one track to another. This excitation will cause the actuator to resonate at its natural frequency. If a drive fails to seek and settle down on the desired track, the drive may be experiencing a resonance.
A drive can also be tested for resonance symptoms by increasing the gain of a servo system. The servo system generally provides a servo control to stabilize the W/R head over a track during a read/write operation. The gain of the servo system controls the settling time of the head when the drive seeks to a track. For a certain system, the gain of the servo system affects the stability of the servo systemxe2x80x94the higher the open loop gain of the servo system, the smaller the gain margin. With a higher servo gain, the drive is more prone to any resonance mode as its open loop gain margin will be lowered. With a smaller gain margin, the open loop gain response of a VCM resonance frequency region is more likely to reach the 0-dB point, causing the actuator to oscillate. When the actuator oscillates, the time to settle is much longer. By observing the settling time, drives with resonance can be detected.
The existing typical resonance screening system used at the manufacturing stage of a disc drive is a xe2x80x9cDynamic Signal Analyzerxe2x80x9d by Hewlett Packard (HP). This device can be used to get the open loop bode plot of a control system. From the bode plot, we can tell whether the control system has resonance and also at which frequency. Although this analyzer detects the actuator resonance, it can only be used off-line and is very time consuming. It is only used in the design phase and failure analysis phase to verify whether the actuator really has a resonance problem.
Also, in current practice, once a drive is determined to exhibit actuator resonance, the entire drive is rejected. However, a drive does not necessarily exhibit resonance at all regions across the disc. Thus, the current practice of rejecting a drive with very low resonance may lower the production yield unnecessarily.
The present invention provides a solution to the above and other problems and offers advantages over the prior solutions to the above and other problems.
The present invention provides a disc drive system, a method, or an apparatus for detecting drive resonance and compensating the drive with a symptom of resonance.
In general terms, the present invention provides a resonance screening system by establishing a multi servo zone architecture. More particularly, the present invention provides a resonance screening system by partitioning a disc into several servo zones. By partitioning the disc into several servo zones, each with its associated gain will prevent drives, especially the drives with very low resonance from being scrapped. One of the ways of doing so is to reduce the servo gain of the region where resonance occurs. Reducing a servo gain increases an open loop gain margin for that weak zone, i.e., the zone with a resonance. One of the resonances which can be screened in the present invention is actuator resonance.
One of the advantages of the present invention is that by partitioning the disc into several servo zones, a drive with little or very low actuator resonance at a certain zone does not affect a servo bandwidth of the other good zones, i.e., the zones without actuator resonance. The servo gain of the good zones does not have to be adjusted. Thus, the drive with very low resonance is not rejected, thereby increasing the production yield of the drive.
Accordingly, the present invention provides a cost-effective resonance screening technique. The present resonance screening technique significantly reduces the number of rejected drives at the manufacturing stage, thus improving the production yield. The present resonance screening technique also ensures that a drive which passes the resonance detection test has an adequate servo margin for its normal operation after going through the test.